My cat is also both! We are lucky enough to live in such a place where it's safe for her to be outside. We have a house with a big yard and nice neighbours in a semi rural area. She spends about 50% of her time inside and 50% outside. Well, more time outside during the summer of course. She goes in and out whenever we open the front or back door (when letting the dogs out for a pee for example), or she sits and scratches the door to get out/in. She always stays close, either outside our house or the house next door. Apparently, they told us, during the summers when they keep their door open, she sometimes sneaks in and they find her snoozing on their bed! Cheeky cat.
But yeah, it has never been a problem here, and she always comes back for food and to sleep as she prefers sleeping inside. BUT - if I would live in any other condition and with a new cat, I wouldn't have it like this. She's 12 years old now and isn't exactly going to go anywhere. But when I do move away from home and into a flat, and if I do get another cat, it would be an indoor one.